Always question .Pre-heresy? Where's that from? Not questioning it, just curious.
It's from Battle of Gate Forty-Two. It occurred after Horus's rise to Warmaster but before he was turned to Chaos.
Horus used the combined might of the Luna Wolves, Space Wolves, Iron Warriors, and the Raven Guard to crush a human rebellion on some formerly compliant worlds. Horus had all four Legions blitz the enemy "lair", as it is described. The quick war was done in hopes of protecting the infrastructure of the systems against a long-term war.
Corax felt it was unwise to use his troops in such a blunt matter...Horus and Perturabo thought otherwise. The attack went through. Corax lost a lot of men in a style of war he really didn't like.
He swore he'd never serve under Horus again.